2. ALGORITHMS AND FLOWCHARTS
A typical programming task can be divided into
two phases:
Problem solving phase
produce an ordered sequence of steps that describe
solution of problem
this sequence of steps is called an algorithm
Implementation phase
implement the program in some programming
language
3. Steps in Problem Solving
First produce a general algorithm (one can use
pseudocode)
Refine the algorithm successively to get step by
step detailed algorithm that is very close to a
computer language.
Pseudocode is an artificial and informal
language that helps programmers develop
algorithms. Pseudocode is very similar to
everyday English.
4. Pseudocode & Algorithm
Example 1: Write an algorithm to
determine a student’s final grade and
indicate whether it is passing or failing.
The final grade is calculated as the
average of four marks.
5. Pseudocode & Algorithm
Pseudocode:
Input a set of 4 marks
Calculate their average by summing and dividing
by 4
if average is below 50
Print “FAIL”
else
Print “PASS”
7. The Flowchart
(Dictionary) A schematic representation of a sequence of
operations, as in a manufacturing process or computer
program.
(Technical) A graphical representation of the sequence
of operations in an information system or program.
Information system flowcharts show how data flows from
source documents through the computer to final
distribution to users. Program flowcharts show the
sequence of instructions in a single program or
subroutine. Different symbols are used to draw each
type of flowchart.
8. The Flowchart
A Flowchart
shows logic of an algorithm
emphasizes individual steps and their
interconnections
e.g. control flow from one action to the next
9. Flowchart Symbols
Basic
Oval
Parallelogram
Rectangle
Diamond
Hybrid
Name Symbol Use in Flowchart
Denotes the beginning or end of the program
Denotes an input operation
Denotes an output operation
Denotes a decision (or branch) to be made.
The program should continue along one of
two routes. (e.g. IF/THEN/ELSE)
Denotes a process to be carried out
e.g. addition, subtraction, division etc.
Flow line Denotes the direction of logic flow in the program
10. Example
PRINT
“PASS”
Step 1: Input M1,M2,M3,M4
Step 2: GRADE (M1+M2+M3+M4)/4
Step 3: if (GRADE <50) then
Print “FAIL”
else
Print “PASS”
endif
START
Input
M1,M2,M3,M4
GRADE(M1+M2+M3+M4)/4
IS
GRADE<5
0
PRINT
“FAIL”
STOP
YN
11. Example 2
Write an algorithm and draw a flowchart to
convert the length in feet to centimeter.
Pseudocode:
Input the length in feet (Lft)
Calculate the length in cm (Lcm) by
multiplying LFT with 30
Print length in cm (LCM)
13. Example 3
Write an algorithm and draw a flowchart that
will read the two sides of a rectangle and
calculate its area.
Pseudocode
Input the width (W) and Length (L) of a rectangle
Calculate the area (A) by multiplying L with W
Print A
14. Example 3
Algorithm
Step 1: Input W,L
Step 2: A L x W
Step 3: Print A
START
Input
W, L
A L x W
Print
A
STOP
15. Example 4
Write an algorithm and draw a flowchart that
will calculate the roots of a quadratic equation
Hint: d = sqrt ( ), and the roots are:
x1 = (–b + d)/2a and x2 = (–b – d)/2a
2
0ax bx c
2
4b ac
16. Example 4
Pseudocode:
Input the coefficients (a, b, c) of the
quadratic equation
Calculate d
Calculate x1
Calculate x2
Print x1 and x2
17. Example 4
Algorithm:
Step 1: Input a, b, c
Step 2: d sqrt ( )
Step 3: x1 (–b + d) / (2 x a)
Step 4: x2 (–b – d) / (2 x a)
Step 5: Print x1, x2
START
Input
a, b, c
d sqrt(b x b – 4 x a x c)
Print
x1 ,x2
STOP
x1 (–b + d) / (2 x a)
X2 (–b – d) / (2 x a)
4b b a c
18. DECISION STRUCTURES
The expression A>B is a logical expression
it describes a condition we want to test
if A>B is true (if A is greater than B) we take
the action on left
print the value of A
if A>B is false (if A is not greater than B) we
take the action on right
print the value of B
23. Example 5
Write an algorithm that reads two values, determines the
largest value and prints the largest value with an
identifying message.
ALGORITHM
Step 1: Input VALUE1, VALUE2
Step 2: if (VALUE1 > VALUE2) then
MAX VALUE1
else
MAX VALUE2
endif
Step 3: Print “The largest value is”, MAX
24. Example 5
MAX VALUE1
Print
“The largest value is”,
MAX
STOP
Y N
START
Input
VALUE1,VALUE2
MAX VALUE2
is
VALUE1>VALUE2
25. NESTED IFS
One of the alternatives within an IF–
THEN–ELSE statement
may involve further IF–THEN–ELSE
statement
26. Example 6
Write an algorithm that reads three
numbers and prints the value of the largest
number.
27. Example 6
A Step 1: Input N1, N2, N3
B Step 2: if (N1>N2) then
c if (N1>N3) then
d MAX N1 [N1>N2, N1>N3]
e else
f MAX N3 [N3>N1>N2]
g endif
h else
i if (N2>N3) then
j MAX N2 [N2>N1, N2>N3]
k else
l MAX N3 [N3>N2>N1]
m endif
n endif
O Step 3: Print “The largest number is”, MAX
29. Example 7
Write and algorithm and draw a flowchart
to
a) read an employee name (NAME),
overtime hours worked (OVERTIME),
hours absent (ABSENT) and
b) determine the bonus payment
(PAYMENT).
30. Example 7
Bonus Schedule
OVERTIME – (2/3)*ABSENT Bonus Paid
>40 hours
>30 but 40 hours
>20 but 30 hours
>10 but 20 hours
10 hours
$50
$40
$30
$20
$10
31. Step 1: Input NAME,OVERTIME,ABSENT
Step 2: if (OVERTIME–(2/3)*ABSENT > 40) then
PAYMENT 50
else if (OVERTIME–(2/3)*ABSENT > 30) then
PAYMENT 40
else if (OVERTIME–(2/3)*ABSENT > 20) then
PAYMENT 30
else if (OVERTIME–(2/3)*ABSENT > 10) then
PAYMENT 20
else
PAYMENT 10
endif
Step 3: Print “Bonus for”, NAME “is $”, PAYMENT
